The foundation
Published math, worked the standard way.
Our calculators use the same formulas taught in finance courses and textbooks: compound growth, loan amortization, interest paid, monthly payments, present and future value, expense ratios, payoff schedules, and refinance comparisons. Where a standard formula exists, we use it. The calculations are deterministic — the same inputs always produce the same outputs, with no hidden randomness — so a result you can get today, you can reproduce tomorrow.
Transparency
Assumptions are visible and, where practical, editable.
A calculator is only as good as the assumptions behind it. We surface those assumptions as inputs on the page whenever it is practical to do so, with reasonable, clearly labeled defaults. That means you are never stuck with a number we picked for you: change the interest rate, the time horizon, or the contribution amount, and watch the result move. Seeing that movement is the point — it is how you learn which assumptions matter most.
Assumptions in detail
Inflation.
Because a dollar buys less over time, several tools let you view results in real (inflation-adjusted) terms as well as nominal dollars. When we adjust for inflation, we divide future dollars by compounding price growth at the rate you choose. Example: at 3% inflation, $10,000 today has about $10,000 ÷ 1.0310 ≈ $7,441 of purchasing power in ten years. The inflation rate is an assumption you set, not a prediction.
Investment returns.
Investment returns are assumptions, not guarantees. Where a tool illustrates a growth rate, that rate is an input you control. Where a tool backtests against history, it uses the public market series described on our Data Sources page — a broad U.S. market total-return series, not a single index or a hand-picked period. Past performance does not predict future returns, and we say so on the pages where it matters.
Loans.
Loan tools use standard fixed-rate amortization. Example: a $20,000 loan at 6% over 60 months has a monthly payment of about $386.66 and totals roughly $23,200 — about $3,200 of interest. Stretch the same loan to 72 months and the monthly payment falls, but the total interest rises. The math is exact; the rate and term you enter are the assumptions.
Taxes.
Some tools apply tax figures — for example, retirement-contribution limits or a marginal tax rate. These are simplified: they use published reference figures (such as IRS contribution limits) and a rate you provide, and they do not attempt to model your entire tax return, state taxes, phase-outs, or every credit and deduction. Treat tax outputs as illustrations, not tax advice.
Limits
Small assumptions, large differences.
Results can be highly sensitive to their assumptions, especially over long horizons. Example: $10,000 growing for 30 years is about $57,435 at 6% but about $76,123 at 7% — a single percentage point changes the ending balance by roughly a third. That sensitivity is exactly why the assumptions are visible: a calculator's job is to show you the shape of a decision, not to promise a specific dollar amount. These tools are educational, not financial advice, and they cannot account for your full circumstances.
How we check the math
Reviewed and tested.
Because the calculation logic is kept separate from the interface wherever practical, it can be checked against known formulas and worked examples. Our Calculator Testing page explains how we do that — and why we still ask you to report anything that looks off. For the standards behind all of this, see our Editorial Policy.
Related reading: Data Sources · Calculator Testing · Editorial Policy.